Organic semiconductors with dithienofuran core monomers

ABSTRACT

An organic semiconducting donor-acceptor (D-A) small molecule, as well as a semiconductor device that can incorporate the D-A small molecule, are disclosed. The D-A small molecule can have electron deficient substituents and R group substituents that can be C 1 -C 20  linear alkyl chains, C 2 -C 24  branched alkyl chains, hydrogen atoms, etc. The D-A small molecule can be can be synthesized in a reaction between a dithienofuran (DTF) core monomer and an electron deficient monomer. Additionally, the D-A small molecule can be part of an organic semiconducting copolymer. A semiconductor device that can incorporate the D-A small molecule in a photoactive layer is also disclosed herein. Additionally,  3,4 -dibrominated furan compound that can, in some embodiments, be a precursor for the D-A small molecule is disclosed. The  3,4 -dibrominated furan compound can be synthesized in a reaction involving a furan- 2,5 -dicarboxylic dimethyl ester (FDME), which can have a bio-renewable precursor.

BACKGROUND

The present disclosure relates to semiconducting organic materials and,more specifically, electron donor-acceptor copolymers and smallmolecules with substituted dithienofuran core monomers.

Organic semiconducting polymers and small molecules have manyapplications in electronic devices. Organic semiconductors differ frominorganic semiconductors, such as silicon, in a number of ways. One wayis that organic semiconductors can be less expensive than inorganicsemiconductors. Organic semiconductors can also be flexible, allowingthem to be incorporated into a wide variety of materials andtechnologies. Some organic semiconductors can be used in optoelectronicdevices, such as solar cells or light sensors. Organic semiconductorsknown as organic photovoltaic (OPV) materials, can produce an electricalcurrent upon exposure to photons. Organic semiconductors can be producedfrom non-renewable petroleum-based resources or bio-renewable sources.

SUMMARY

Various embodiments are directed to organic semiconductingdonor-acceptor (D-A) small molecules and semiconductor devices that canincorporate the organic semiconducting D-A molecules. The D-A smallmolecules can be substituted dithienofuran (DTF) core monomers bound toelectron deficient monomers. Substituents on the DTF core monomer can beR groups such as C₁-C₂₀ linear alkyl chains, a C₂-C₂₄ branched alkylchains, etc. In some embodiments, a 3,4-dibrominated furan compound canbe a precursor of the DTF core monomers. The 3,4-dibrominated furancompound can be synthesized in a reaction involvingfuran-2,5-dicarboxylic dimethyl ester (FDME), which can have abio-renewable precursor. Examples of bio-renewable precursors caninclude sugars and aldaric acids.

The D-A small molecule can be synthesized in a reaction between a DTFcore monomer and an electron deficient monomer. Examples of electrondeficient monomers that can be bound to the DTF core, providing the D-Asmall molecule with electron deficient substituents, can includebromoalkylthienyl-pyridylthiazoles, benzodithiazoles,pyridyldithiazoles, diketopyrrolopyrroles, dithienopyrrolodiones,thienothiophene esters, fluorinated thienothiophene esters,dithienotetrazines, thienoquinoxalines, benzoquinoxalines,pyridylquinoxalines, etc. The D-A small molecule can also be part of anorganic semiconducting copolymer with repeating units of the DTF coremonomer and electron deficient monomer.

The organic semiconducting D-A small molecules and copolymers can be inthe semiconducting, photoactive layers of semiconductor devices. Thesedevices can include a first and a second electrode, which can compriseat least one conductive material, such as a metal, a metal oxide, ametal alloy, a conductive polymer, Ag nanowires, Cu nanowires, graphene,carbon nanotubes, a polymer-metal hybrid, carbon-sulfur nanotubes,nanofibers, an organic polymer, etc. The semiconducting layer can be abilayer or a bulk heterojunction, and it can include materials inaddition to the D-A small molecules or copolymers, such as fullerenes,polymers, and polymer blends. Examples of semiconductor devices that canincorporate the organic semiconducting compounds can include organicphotovoltaic cells (OPVs), field effect transistors (FETs), lightsensors, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic summary of steps in processes of forming adibrominated furan compound and a dithienofuran compound, according tosome embodiments.

FIG. 1B is a diagrammatic summary of steps in a process of forming anorganic semiconducting small molecule, according to some embodiments.

FIG. 1C is a diagrammatic summary of steps in a process of forming anorganic semiconducting copolymer, according to some embodiments.

FIG. 2 is a chemical reaction diagram illustrating a process ofsynthesizing a bis-alkylketone furan and a dibrominated furan compound,according to some embodiments.

FIG. 3 is a diagrammatic representation of examples of bio-renewablecompounds that can be involved in the production offuran-2,5-dicarboxylic dimethyl ester (FDME) according to someembodiments.

FIG. 4 is a chemical reaction diagram illustrating a process ofsynthesizing a substituted dibromo dithienofuran compound, according tosome embodiments.

FIG. 5 is a chemical reaction diagram illustrating a process ofsynthesizing a bis-boronic ester-DTF monomer, according to someembodiments.

FIG. 6 is a chemical reaction diagram illustrating a process ofsynthesizing a bis-trialkylstannyl-dithienofuran monomer, according tosome embodiments.

FIG. 7 is a diagrammatic representation of a dithienofuran (DTF) coremonomer and three of its optional substituents, according to someembodiments.

FIG. 8 is a chemical reaction diagram illustrating a process ofsynthesizing a donor-acceptor small molecule, according to someembodiments.

FIG. 9 is a chemical reaction diagram illustrating a process ofsynthesizing a donor-acceptor copolymer, according to some embodiments.

FIG. 10 is a flow diagram illustrating a process of electron excitationand transfer triggered by photon absorption by an organic semiconductor,according to some embodiments.

FIG. 11A is a diagrammatic illustration of an exemplary organicphotovoltaic (OPV) cell, according to some embodiments.

FIG. 11B is a diagram illustrating a process of electron transfer in anorganic semiconductor, according to some embodiments.

DETAILED DESCRIPTION

FIGS. 1A, 1B, and 1C summarize steps in the syntheses of compoundsdiscussed herein, according to some embodiments. Examples of thesesynthetic processes will be discussed in greater detail in FIGS. 2, 4,5, 6, 8, and 9.

FIG. 1A is a diagrammatic summary of steps in processes 100-1 and 100-2of forming, respectively, a dibrominated furan compound 130 and adithienofuran compound 140, according to some embodiments. The firstdiagram illustrates a process 100-1 of converting furan-2,5-dicarboxylicdimethyl ester (FDME) 110 into a 3,4-dibrominated furan compound 130,according to some embodiments. FDME can come from a bio-renewableprecursor before becoming involved in the synthesis of compound 130. The3,4-dibrominated furan compound 130 can be involved in the synthesis oforganic semiconducting compounds discussed herein, according to someembodiments. Details of the synthesis of compound 130 will be discussedin FIG. 2.

Also shown in FIG. 1A is a summary of a process 100-2 of converting the3,4-dibrominated furan compound 130 into a dibromo dithienofuran (DTF)compound 140, according to some embodiments. The dibromo-DTF monomer 140and its derivatives can be monomers in semiconducting organic smallmolecules and copolymers, as discussed below. The dibromo-DTF monomer140 shown in FIG. 1A has two substituents in addition to its bromosubstituents. These are both labeled R, though these substituents do notnecessarily need to be identical substituents. R can represent varioussubstituents, including alkanes, alkenes, fluorinated alkanes, ringstructures with or without heteroatoms, etc., as will be discussed ingreater detail below. Details of the synthesis of the dibromo-DTFmonomer 140 will be discussed with respect to FIG. 4.

FIG. 1B is a diagrammatic summary of steps in a process 100-3 of formingan organic semiconducting small molecule 150, according to someembodiments. As indicated here, the dibromo-DTF monomer 140 can be abuilding block in the synthesis of an organic semiconducting smallmolecule 150. In some examples, the dibromo-DTF monomer 140 and itsderivatives can react with at least one monomer having greater electronaffinity. In FIG. 1B, this is referred to as the “electron deficientmonomer (E)” 145. Small molecules with monomers of different electronaffinities can be called donor-acceptor (D-A) small molecules 150,wherein the “acceptor” monomer has a greater electron affinity than the“donor” monomer and, due to this property, can accept electrons from themore electron rich donor. The exemplary D-A small molecule 150illustrated here combines one donor monomer, the dibromo-DTF monomer140, and two equivalents of an acceptor (electron deficient) monomer145. Details of an example of the synthesis of a D-A small molecule arediscussed with regard to FIG. 8.

FIG. 1C is a diagrammatic summary of steps in a process 100-4 of formingan organic semiconducting copolymer 160, according to some embodiments.The dibromo-DTF monomer 140 can be a building block in the synthesis ofan organic semiconducting copolymer 160, as it was in the process 100-3of forming the D-A small molecule 150 in FIG. 1B. In some examples, thedibromo-DTF monomer 140 and its derivatives can react with an electrondeficient monomer 145. Polymers with alternating monomers of differentelectron affinities can be called donor-acceptor (D-A) copolymers 160,wherein the “acceptor” monomer has a greater electron affinity than the“donor” monomer and, due to this property, can accept electrons from themore electron rich donor. The exemplary D-A copolymer 160 shown herecomprises repeating units of one donor monomer, the dibromo DTF monomer140, and one acceptor (electron deficient) monomer 145. Details of anexample of the synthesis of a D-A copolymer are discussed with respectto FIG. 9.

It should be understood that the chemical reaction diagrams illustratedherein are prophetic examples and are not limiting; they can vary inreaction conditions, components, methods, etc. The details of syntheticprocesses described below include reaction conditions (e.g.,temperature, time, solvent, identity of inert gases, methods, etc.) andreagents, but these are simply examples and can vary in otherembodiments. In addition, reaction conditions can optionally be changedover the course of a process. Further, in some embodiments, processescan be added or omitted while still remaining within the scope of thedisclosure, as will be understood by a person of ordinary skill in theart.

FIG. 2 is a chemical reaction diagram illustrating a process 100-1 ofsynthesizing a bis-alkylketone furan compound 220 and a 3,4-dibrominatedfuran compound 130, according to some embodiments. Process 100-1 wasintroduced in FIG. 1A, and an example of this process is described ingreater detail here. FIG. 2 illustrates process 100-1 as broken into twoparts. The first part illustrates the conversion offuran-2,5-dicarboxylic dimethyl ester (FDME) 110 to a bis-alkylketonefuran compound 220. In some embodiments, FDME 110 can optionally beobtained by synthesis reactions involving bio-renewable compounds,though this is not necessary. Bio-renewable compounds are compounds thatcan be obtained from renewable sources, such as plant products, animalproducts, microorganisms, etc. Examples of bio-renewable compounds areillustrated in FIG. 3.

Process 100-1 can begin by converting FDME 110 to a bis-alkylketonefuran 220 via a carbon-carbon bond formation reaction, according to someembodiments. In some embodiments, process 100-1 can be accomplished byreacting FDME 110 with a Grignard or lithium reagent. A Grignard reagentcomprises an alkyl-, vinyl-, or aryl-magnesium halide. Process 100-1illustrates the use of a Grignard reagent, RMgBr, where R can be analkyl group, though the identities of optional R groups will bediscussed in greater detail below. Alkyl groups include hydrocarbonshaving one or more carbon atoms that are connected to one another bysingle bonds. Grignard reactions can take place in the presence of oneor more ethereal solvents, such as tetrahydrofuran (THF), diethyl ether,di-tert-butyl ether, etc. In addition, the Grignard reaction can bequenched by a proton source. Protons are generally written as H⁺ inchemical reaction diagrams, and when they are in the presence of water(e.g., in an aqueous environment), they are often written as H⁺/H₂O orH₃O⁺. In FIG. 2, the proton source in process 100-1 is denoted H₃O⁺, butthis should not be taken to limit the proton source to aqueousenvironments, or to suggest that the Grignard reaction necessarily takesplace in the presence of water.

One example of how the conversion of FDME 110 to a bis-alkylketone furan220, as illustrated in FIG. 2, can be carried out is by adding a THFsolution of an alkylmagnesium bromide, denoted RMgBr in FIG. 2, to asolution of FDME 110. The mixture can then be stirred at low temperature(e.g., −78° C.) before being quenched by a proton source, such as anaqueous solution of HCl. The aqueous layer can then be extracted with anorganic solvent, such as diethyl ether. A drying agent, such asmagnesium sulfate, can be added to the combined organic phases in orderto remove any additional water left in the solution. Following this, themixture can be concentrated and purified by column chromatography. Thepurified sample can contain the bis-alkylketone furan 220.

In some cases, over-alkylation of FDME 110 can occur during process100-1. If this occurs, the FDME can be reduced to a bis-aldehyde furan210 using a hydrogen atom source. Two examples of hydrogen atom sourcesthat can be used in this reaction are diisobutylaluminium hydride(DIBAL-H) and sodium bis(2-methoxyethoxy)aluminiumhydride (Red-Al),though others can be employed. Compound 210 can then be transformed intocompound 220 using any of the possible methods that can be used toconvert compound 110 to compound 220. In the exemplary process 100-1illustrated in FIG. 2, the same conditions are used in both reactions,but this need not be the case. Further, the conversion of compound 210to compound 220 can optionally be carried out at higher temperaturesthan the conversion of compound 110 to compound 220. For example,compound 210 can be converted to compound 220 at 0° C.

The process 100-1 illustrated in FIG. 2 can include forming a3,4-dibrominated bis-alkylketone furan compound 130. In this example,the bis-alkylketone 220 can be converted to a 3,4-dibrominatedbis-alkylketone furan 130 by a bromination reaction. This can beaccomplished through various methods. One of these, shown in FIG. 2, isthe reaction of the bis-alkylketone 220 with a2,2,6,6-tetramethylpiperidinyl magnesium chloride lithium chloridecomplex (TMPMgCl·LiCl), followed by the addition of an electrophilicbromine source, generically denoted [Br]⁺. Electrophilic bromine cancome from various sources, including elemental bromine (Br₂),N-bromosuccinimide (NBS), benzenesulfonyl bromide, dibromoethane,tetrabromoethane, hexabromoethane, sulfonyl bromides, and others. Theaddition of [Br]⁺ can result in the binding of one or more brominesubstituents to compound 220, yielding the 3,4-dibrominatedbis-alkylketone furan 130.

One example of how the process of forming the 3,4-dibrominatedbis-alkylketone furan 130 can be carried out is to add, dropwise and atlow temperature (e.g. −78° C.), an anhydrous solution of thebis-alkylketone 220 to a THF solution of TMPMgCl·LiCl. This addition iscarried out under an inert gas such as argon or nitrogen. The mixture isthen stirred at low temperature for approximately an hour prior to theaddition of an electrophilic bromine source. Upon addition of theelectrophilic bromine, the mixture is stirred at low temperature forlonger than an hour and then allowed to warm to room temperature (i.e.,a temperature ranging from 15° C. to 30° C.). Once at room temperature,the reaction is quenched with a saturated aqueous solution of ammoniumchloride. The aqueous layer is extracted with an organic solvent, andadditional water is removed from the mixture by adding a drying agent,such as magnesium sulfate. The mixture is concentrated and then purifiedby column chromatography.

FIG. 3 is a diagrammatic representation of examples of bio-renewablecompounds that can be involved as precursors in the production of FDME110, according to some embodiments. One of these is fructose 305, asugar that can be produced through photosynthesis and is found in manyplants. Other examples include aldaric acids, which are polyhydroxydicarboxylic acids that can be derived from aldoses through oxidation atboth terminal carbon atoms. D-Glucaric acid 310 is one example of analdaric acid, but this is for illustrative purposes, and other aldaricacids can be used. It is possible to derive an aldaric acid from anaturally occurring aldose, though this is not necessary. Anotherexample involves an enzyme, such as oxidoreductase HmfH, that can reactwith molecular oxygen to produce a precursor for FDME 110,furan-2,5-dicarboxylic acid (FDCA) 315. It should be understood thatthese bio-renewable substances are only examples, and other compoundscan be used in the production of FDME. Further, the compounds discussedhere, as well as other compounds involved in the production of FDME, canbe obtained from sources that are not bio-renewable. Additionally, FDMEcan be obtained commercially or through other means.

FIG. 4 is a chemical reaction diagram illustrating a process 100-2 ofsynthesizing a substituted dibromo dithienofuran (DTF) compound 140,according to some embodiments. In this example, the dibromo DTF compound140 has alkyl substituents. Process 100-2 was introduced in FIG. 1A, andan example of this process is described in greater detail here. A firststep in process 100-2 yields a bis-alkyl-DTF-diester fused ringstructure 410 in a cyclization reaction, and a second step in process100-2 yields a dibromo bis-alkyl-DTF monomer (a dibromo-DTF compound)140.

The process 100-2 can involve two steps. In one example of how the firststep of process 100-2 can be carried out, the 3,4-dibrominated furan 130is mixed with potassium carbonate (K₂CO₃) in the solvent DMF. To thismixture, ethyl-mercaptoacetate is added dropwise at a temperature warmerthan room temperature (e.g., 60° C.). The mixture is stirred forapproximately 48 hours under an inert gas, such as nitrogen or argon.The mixture is then poured into water and the organic and aqueous layersare allowed to separate. The aqueous layer is extracted with ethylacetate, and the organic layer is washed with water and brine (aconcentrated aqueous solution of a salt, such as sodium chloride). Wateris removed from the organic layers by adding a drying agent, such asmagnesium sulfate. The solvent is then evaporated, and the residuepurified by column chromatography.

Compound 410 has two ester substituents, and they can be replaced withhalogen atoms (e.g., bromine or iodine) during a second step in process100-2, according to some embodiments. An example of a process ofreplacing the ester substituents with bromo substituents is illustratedin FIG. 4. In this example, a base (LiOH) is added to a THF solution ofthe bis-alkyl DTF-diester compound 410. An electrophilic bromine source,such as N-bromosuccinimide (NBS), is then added and the mixture isstirred for approximately 12 hours at room temperature (i.e., 15° C. to30° C.). In some embodiments N-iodosuccinimide can be used in place ofNBS, resulting in iodo substituents in the place of bromo. The mixtureis then extracted with an organic solvent, such as dichloromethane. Theorganic layers are washed with a saturated aqueous solution of sodiumbicarbonate, followed by water, and then brine. Water is removed fromthe organic layers by adding a drying agent, such as magnesium sulfate.The mixture is concentrated and then added to a polar solvent, such asethanol, in order to form a precipitate. The precipitate is purified byrecrystallization. In some embodiments, process 100-2 can result in theformation of an alkylated dibromo-DTF monomer 140, an example of whichwas introduced in FIG. 1A.

FIG. 4 illustrates one example of a synthesis of the alkylateddibromo-DTF monomer 140 but, in other embodiments, a compound such asthis can be obtained through alternate processes. One example of analternate route to compound 140 can occur via decarboxylation ofcompound 410 using copper powder in quinoline at a high temperature(e.g., 250° C.), followed by bromination. Further, this process can becarried out under varying reaction conditions or with other reagents.

FIG. 5 is a chemical reaction diagram illustrating a process 500 ofsynthesizing a bis-boronic ester-DTF monomer 510, according to someembodiments. This compound is one of several that can be used tosynthesize an organic semiconducting D-A small molecule or copolymer.The process 500 of forming the bis-boronic ester-DTF monomer 510 caninvolve the reaction of an alkylated dibromo-DTF monomer 140 withbis(pinacolato)diboron. In this example, the process 500 is assisted bya palladium catalyst, [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium], abbreviated Pd(dppf)Cl₂, at a high temperature(e.g., 100° C.) in the presence of potassium acetate (KOAc). Othercatalysts, reagents, or reaction conditions can be used in someembodiments.

FIG. 6 is a chemical reaction diagram illustrating a process 600 ofsynthesizing a bis-trialkylstannyl-dithienofuran (DTF) monomer 610,according to some embodiments. Like the bis-boronic ester-DTF monomer510 discussed with respect to FIG. 5, this compound 610 is one ofseveral that can be used to synthesize an organic semiconducting D-Asmall molecule 150 or copolymer 160. In the exemplary process 600,n-butyllithium is added to the dibromo-DTF monomer 140 at lowtemperature (e.g., −78° C.) under an inert gas, such as argon ornitrogen. In one approach, the mixture is stirred for longer than onehour at 0° C. A solution of trimethylstannane chloride, (CH₃)₃SnCl isadded, and the mixture is allowed to reach room temperature (i.e., 15°C. to 30° C.). The mixture is stirred at room temperature forapproximately 12 hours before being combined with water. The organic andaqueous layers are then separated, and the aqueous layer is extractedwith an organic solvent. Additional water can be removed from themixture by adding a drying agent, such as magnesium sulfate. The mixturecan be concentrated, and residual (CH₃)₃SnCl can be removed by heatingthe mixture under reduced pressure (e.g., under vacuum). Process 600 cantake place in various solvents, such as ethers and THF. Other catalysts,reagents, or reaction conditions can be used in some embodiments.

FIG. 7 is a diagrammatic representation of a dithienofuran (DTF) monomer710 and three of its optional substituents, according to someembodiments. The DTF monomer 710 forms the core of the DTF monomersdisclosed herein. The DTF core monomer 710 illustrated here has foursubstituents, two labeled “Z” and two labeled “R.” Examples of possibleR substituents can include hydrogen atoms, alkanes, alkenes, fluorinatedalkanes, etc. The Z substituents can vary and the examples shown in FIG.7 correspond to the substituted DTF monomers 140, 510, and 610, thesyntheses of which were described with respect to FIGS. 4, 5, and 6,respectively. In some embodiments, a DTF monomer can have non-identicalR or Z groups (e.g., a monomer can have two different alkyl Rsubstituents).

There are many examples of R groups that can be substituents on the DTFcore monomer 710. One possible class of R group substituents ishydrocarbons. Hydrocarbons can be saturated or unsaturated. A saturatedhydrocarbon can be an alkyl group. Alkyl groups can includestraight-chain, branched, and cyclic alkanes. Alkanes can include anyhydrocarbon having one or more carbon atoms, wherein the carbon atomsare connected to one another by single bonds and, in most cases, arebound to one or more hydrogen atoms. Cyclic alkanes are those thatinclude one or more rings, which can have three or more carbon atoms.Some examples of R groups can include, but are not limited to, C₁-C₂₀linear alkyl chains, C₂-C₂₄ branched alkyl chains, monoalkyl aminescomprised of C₂-C₂₀ linear alkyl chains or C₁-C₂₄ branched alkyl chains,dialkylamines comprised of C₁-C₂₀ linear alkyl chains, C₁-C₂₄ branchedalkyl chains, etc. Alkyls can also be perfluorinated, wherein hydrogenatoms are replaced with fluorine atoms.

The Z substituents in FIG. 7 are those found on the exemplary DTFmonomers 140, 510, and 610. As discussed above, compound 140 is adibromo-DTF monomer, where Z is a bromo substituent; compound 510 is abis(boronic ester)-DTF monomer, where Z is a boronic ester substituent;and compound 610, is a bis(trimethylstannyl)-DTF monomer, where Z is atrimethylstannyl substituent. Z can also be a number of othersubstituents, and their identity can be chosen based on how easily theycan be cross-coupled to electron deficient monomers duringpolymerization reactions and reactions to form small molecules.

Each of the compounds illustrated in FIG. 7 is a prophetic example of amonomer that can be used in the processes of forming organicsemiconducting electron donor-acceptor (D-A) small molecules 820 andcopolymers 920 illustrated in FIGS. 8 and 9. An electron D-A compound isone that has regions of varying electron affinities. A region withgreater electron affinity is known as an “acceptor” because it is morelikely to accept an electron. Molecules or regions that act as electronacceptors are also known as “electron deficient.”

FIG. 8 is a chemical reaction diagram illustrating a process 100-3,which was introduced in FIG. 1B, of synthesizing a donor-acceptor (D-A)small molecule 820, according to some embodiments. This D-A smallmolecule 820 can act as an organic semiconductor. The example of process100-3 illustrated in FIG. 8 involves a reaction between a DTF coremonomer 710 (e.g., compound 140, 510, or 610) and an electron deficientmonomer. There are many types of electron deficient monomers that can beused; a prophetic example of one of these is from a class of moleculesknown as bromoalkylthienyl-pyridylthiazoles 810, as is shown in FIG. 8.The R substituent on compound 810 can be one of a large variety ofsubstituents, including those listed as possible R substituents on theDTF core monomer 710. Other examples of electron deficient monomers thatcan be used are discussed with respect to FIG. 7. Examples of reactionsthat can be conducted to produce the D-A small molecule 820 include aSuzuki reaction, C-H activation, a Stille reaction, etc.

FIG. 9 is a chemical reaction diagram illustrating process 100-4, whichwas introduced in FIG. 1C, of synthesizing a donor-acceptor (D-A)copolymer 920, according to some embodiments. This D-A copolymer 920 canact as an organic semiconductor. The example of process 100-4illustrated in FIG. 9 involves forming a D-A copolymer 920 from thereaction between a variably substituted, electron rich DTF core monomer710 (e.g., 140, 510, and 610) and an electron deficient aromaticmonomer. In this example, a DTF monomer 710 reacts with an electrondeficient aromatic monomer, a thienopyrrolodione (TPD) compound 910. TheR substituent on compound 810 can one of a large variety ofsubstituents, including those listed as possible R substituents on theDTF core monomer 710. Other electron deficient monomers can be used,including those discussed with regard to FIG. 7. These electrondeficient monomers can be dibromo functionalized or bis-Hfunctionalized. Process 100-4 can produce a D-A copolymer 920 comprisingrepeating units of the combined DTF core monomer 710 andelectron-deficient monomer, TPD 910. Examples of reactions that cancarry out the polymerization in some embodiments can include Suzukicross-coupling polymerization, Stille cross-coupling polymerization, C-Hactivation coupling polymerization, etc.

It should be understood that, in some embodiments, the compoundsdescribed herein can contain one or more chiral centers. These caninclude racemic mixtures, diastereomers, enantiomers, and mixturescontaining one or more stereoisomer. It should be noted that thedisclosed can encompass racemic forms of the compounds as well asindividual stereoisomers, as well as mixtures containing any of these.

The D-A small molecules 150 and copolymers 160 described herein can actas organic semiconductors. Organic semiconductors have numerousapplications and can be used in many of the same applications asinorganic semiconductors. These applications can include semiconductordevices, such as photovoltaic cells (e.g., solar cells), field effecttransistors, light sensors, etc. FIGS. 10, 11A, and 11B illustrateproperties and applications of organic semiconducting small moleculesand copolymers, such as the ones described herein, in a photovoltaicdevice, according to some embodiments. These figures will be discussedtogether.

FIG. 10 is a flow diagram illustrating a process 1000 of electronexcitation and transfer in an organic semiconductor (e.g., D-A smallmolecule 820 or D-A copolymer 920), triggered by photon absorption,according to some embodiments. In this process 1000, energy from lightcan be converted into electricity. In some examples, process 1000 cantake place in an organic photovoltaic (OPV) device or cell, such as cell1100 in FIG. 11A.

FIG. 11A is a diagrammatic representation of an exemplary organicphotovoltaic (OPV) cell 1100, according to some embodiments. An OPV cell1100 can contain one or more types of semiconducting material, which canbe D-A small molecules 150 or D-A copolymers 160. Referring to operation1010 in FIG. 10, these organic semiconducting materials can absorb atleast one photon. An example of photon absorption 1010 by an organicsemiconductor in an OPV cell 1100 is illustrated in FIG. 11A. In thisexample, photons come directly from the sun 1105, though other photonsources can be used (e.g., an ultraviolet (UV) lamp). At least onephoton can travel a path 1110 that arrives at the OPV cell 1100. Thephoton can pass through a transparent electrode 1115 in the OPV cell1100. In this example, the transparent electrode 1115 is an anode.

The exemplary OPV cell 1100 also includes a semiconducting layer 1118between two electrodes, 1115 and 1130. This layer can contain organicmaterials such as the D-A small molecule 820 or the D-A copolymer 920described with respect to FIGS. 8 and 9. In the semiconducting layer1118, there can be at least two regions with different electronaffinities. As in the case of inorganic semiconductors, such as silicon,organic semiconductors can have n-type 1120 and p-type 1122 regions. Ann-type semiconductor is more negatively charged than a p-typesemiconductor because it has an excess of electrons, while a p-typesemiconductor has an excess of positive charges, known as “holes.” Insome embodiments, the n-type material 1120 can comprise a D-A smallmolecule 150 or D-A copolymer 160.

There are a variety of p-type materials 1122 that can be used in thesemiconducting layer 1118 of the OPV cell 1100. Some of these includepolymers, polymer blends, etc. Some examples of p-type polymers arePoly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], commonlyabbreviated MEH-PPV; Poly(9,9-dioctylfluorene-alt-benzothiadiazole),commonly abbreviated F8BT; and poly(3-dodecylthiophene-2,5-diyl)regiorandom, commonly abbreviated P3DDT.

The type of semiconducting layer 1118 illustrated in FIG. 11A is knownas a bilayer, its two layers being n-type 1120 and p-type 1122semiconductors. There are other types of semiconducting layers thatincorporate n-type 1120 and p-type 1122 semiconducting materials, suchas single-layers, bulk heterojunctions, discrete heterojunctions, gradedheterojunctions, etc., and the bilayer shown in FIG. 11A should not betaken as limiting.

In FIG. 10, operation 1010, in addition to encompassing photonabsorption, can involve the generation of what is known as an “exciton.”A photon absorbed by an organic semiconducting material can provideenough energy to excite an electron in the semiconducting layer 1118 ofthe OPV cell 1100 to a higher energy level. When an electron transitionsto a higher energy level, it can be thought of as having left behind apositive charge, known as a “hole.” This electron-hole pair is called anexciton. An example of an exciton 1125 is illustrated in FIG. 11A withinthe semiconducting layer 1118 of the OPV cell 1100.

FIG. 10 also illustrates a charge separation step 1020, wherein theexciton 1125 separates into an electron 1129 and hole 1127 (or negativeand positive charges, respectively). In the example illustrated in FIG.11A, an exciton 1125 is generated in the semiconducting layer 1118,separating into a hole 1127 and an electron 1129. When the exciton 1125is at an interface between n-type 1120 and p-type 1122 semiconductors,the electron 1129 can travel through the n-type semiconductor and thehole 1127 can travel through the p-type semiconducting region.

In the electron transfer operation 1030 of FIG. 10, when the electron1129 leaves behind a positively charged hole 1127, which flows throughthe p-type layer 1120. Operation 1030 will be discussed in greaterdetail with respect to FIG. 11B. FIG. 10 also illustrates an electron1129 and hole 1127 migration step 1040. In this step 1040, the electron1129 in the n-type semiconducting layer 1120 can migrate toward theanode 1115, and the hole 1127 left behind by the transferred electron1129 can migrate toward the cathode 1130. In the OPV cell 1100illustrated in FIG. 11A, the cathode is the counter electrode 1130, andthe anode is the transparent electrode 1115.

Continuing the description of process 1000, illustrated in FIG. 10,electrons 1129 and holes 1127 that have reached the electrodes inoperation 1040 can exit the OPV cell 1100 in operation 1050. Afterexiting through their respective electrodes, electrons 1029 and holes1127 can travel through external wires in order to recombine. As seen inFIG. 11A, the flow of electrons begun in operation 1050 can allow acurrent 1140 to flow through the circuit, which can provide the energyto power some external load 1145. FIG. 11B illustrates the electrontransfer 1030 of process 1000 and is discussed in greater detail below.

FIG. 11B is a diagrammatic illustration of a process of electrontransfer 1030 in an organic semiconductor, according to someembodiments. It is well known that many metals can conduct electricity(i.e., transfer electrons), and this is why, historically, metal wires(e.g., copper and aluminum wires) have been used in electricalapplications. Conductive metals are able to conduct electricity becausetheir electrons are delocalized and can thus move freely and easilythough the metal. Other types of inorganic materials, includingmetalloids (e.g., silicon), have a certain degree of electrondelocalization, allowing them to act as semiconductors. Organicmaterials that can conduct electricity, including organicsemiconductors, also rely on electron delocalization. In an organicmaterial, electron delocalization is most easily achieved in what isknown as a “conjugated system.” Conjugated systems include organicmaterials with overlapping molecular orbitals in molecules that havealternating single and double or triple bonds. In a conjugated system,electrons in these molecular orbitals are not localized to a particularbond or atom, but can instead move freely through the overlappingorbitals.

Molecular orbitals can have different energy levels. Some molecularorbitals are occupied by electrons, which fill the orbitals with lowerenergy levels before those with higher energy levels. The highest energyorbital that contains an electron is called the highest occupiedmolecular orbital (HOMO), and the lowest energy orbital that doesn'tcontain an electron is called the lowest unoccupied molecular orbital(LUMO). Because electrons fill orbitals beginning with the lowest energylevels, the LUMO is at the next highest energy level after the HOMO. Theenergy for electron excitation in a conjugated organic system can beprovided by a photon, allowing the excited electron to transition fromthe HOMO to the LUMO. Overlapping LUMOs in a conjugated organicsemiconducting material can be thought of as a “conduction band,”wherein electrons that are excited into the LUMO become delocalized andcan move freely through all overlapping molecular orbitals. This allowsthe conjugated organic material to conduct electrons in a manner similarto metals and metalloids.

Turning to FIG. 11B, molecular orbitals in an organic semiconductingmaterial are illustrated as being at different energy levels. The HOMOsare the lowest energy levels shown, and the LUMOs are the highest energylevels shown. In this example, the electron 1129 has been excited by aphoton 1110 from the sun 1105. Upon excitation, the electron moves fromthe HOMO of the p-type semiconductor 1155 to the LUMO of the p-typesemiconductor 1160, leaving a hole 1127 behind in the p-type HOMO 1155.The electron 1129 is then transferred from the LUMO of the p-typesemiconductor 1160 to the LUMO of the n-type semiconductor 1170 in anelectron transfer step 1030. The hole 1127 then migrates toward theanode 1115 and can leave the cell through this electrode 1115. Inaddition, the transferred electron 1129 can leave the cell through thecathode 1130. The electron 1129 and hole 1127 can then travel through anexternal wire, producing a current 1140 that can power an external load1145.

An organic photovoltaic (OPV) cell, such as the cell 1100 illustrated inFIG. 11A, can incorporate various components and materials in additionto the type semiconducting organic materials discussed herein. Someexamples of these components and materials from which they can be madeare discussed below.

In some embodiments, an OPV cell 1100 has two electrodes. A firstelectrode, e.g., the transparent electrode 1115 in FIG. 11A, can be madeof a material with qualities that include, but are not limited to,transparency and conductivity. Some examples of these materials caninclude metal oxides such as zinc oxides, tin oxides, indium oxides,indium tin oxides, and indium zinc oxides. Additionally, the transparentconducting material can comprise a combination of two or more of thesemetal oxides. Other combinations can be of two or more metals and metaloxides, such as ZnO:Al or SnO₂:Sb. Still other examples can includemetals such as vanadium, chromium, copper, zinc, gold, or an alloythereof. Conductive polymers, such as poly(3-methylthiphene),poly[3,4-(ethylene-1,2-di-oxy)thiophene] (PEDOT), polypyrrole, andpolyaniline, are among others can be used. The electrode 1115 canoptionally be a flexible electrode made of one or more materials thatcan include, but are not limited to, Ag nanowires, Cu nanowires,graphene, carbon nanotubes, polymers, and polymer-metal hybrids.

An OPV cell 1100 also contains a second electrode, e.g., the counterelectrode, or cathode 1130, shown in FIG. 11A. In some embodiments, thiselectrode 1130 can be a metal having a small work function. Examples ofmetals such as this include lithium, sodium, potassium, magnesium,calcium, titanium, indium, yttrium, gadolinium, aluminum, silver, tin,and lead. The metal can also be an alloy. In some embodiments, thecounter electrode 1130 can have a multilayered structure. Examples ofmultilayered structures that can be utilized include LiF/Al, LiO₂/Al,LiF/Fe, Al:Li, Al:BaF₂, and Al:BaF₂:Ba. The second electrode 1130 canalso be made of non-metal or metal-non-metal hybrid materials. Examplesof these include carbon-sulfur nanotubes, nanofibers, and organicpolymers.

As discussed above, an OPV cell 1100 can contain at least onesemiconducting organic material in its semiconductor layer 1118. Thesematerials can include some that were not discussed above. Asemiconducting organic material can be a conjugated organic molecularcompound or compounds, according to some embodiments. Molecules in anOPV cell 1100 can have any level of size or complexity, encompassingmonomers, dimers, trimers, polymers, copolymers, and others. They can becombined with one or more materials that may or may not be conductiveand may or may not be molecular. Some examples of other conductivematerials that can optionally be incorporated are electrolytes,inorganic or ionic substances, metals, some allotropes of carbon, andorganic or organometallic substances.

Organic photovoltaic compounds such as those described herein can have anumber of applications. For instance, an organic semiconducting D-Asmall molecule 150 or copolymer 160 can be utilized in flexible OPVcells. Here, the semiconducting compounds can be deposited onto aflexible substrate, examples of which include paper, fabrics, andsynthetic polymers, such as polyethylene terephthalate (PET). FlexibleOPV cells can be incorporated into items such as clothing, flexibleelectronic screens, etc.

The examples discussed herein and represented in the accompanyingdrawings may make reference to particular details. However, it will beunderstood that there are various modifications that can be made whileretaining the spirit and scope of the disclosure. These would be easilyrecognized and carried out by one of ordinary skill in the art.

What is claimed is:
 1. A semiconducting compound of the formula:

wherein each of R is selected from a group consisting of a C₁-C₂₀ linear alkyl chain, a C₂-C₂₄ branched alkyl chain, and a hydrogen atom; and wherein each of E is an electron deficient substituent.
 2. The semiconducting compound of claim 1, wherein the electron deficient substituent is selected from a group consisting of a bromoalkylthienyl-pyridylthiazole, a benzodithiazole, a pyridyldithiazole, a diketopyrrolopyrrole, a dithienopyrrolodione, a thienothiophene ester, a fluorinated thienothiophene ester, a dithienotetrazine, a thienoquinoxaline, a benzoquinoxaline, and a pyridylquinoxaline.
 3. The semiconducting compound of claim 1, wherein the semiconducting compound is synthesized in a reaction between a dithienofuran core monomer and the electron deficient monomer.
 4. The semiconducting compound of claim 1, wherein the semiconducting compound is part of a semiconducting copolymer.
 5. The semiconducting compound of claim 1, wherein the semiconducting compound is synthesized in a reaction involving a 3,4-dibrominated furan compound.
 6. A 3,4-dibrominated furan compound of the formula:

wherein each of R is selected from a group consisting of a C₁-C₂₀ linear alkyl chain, a C₂-C₂₄ branched alkyl chain, and a hydrogen atom.
 7. The 3,4-dibrominated furan compound of claim 6, wherein the 3,4-dibrominated furan compound is synthesized in a reaction involving a furan-2,5-dicarboxylic dimethyl ester.
 8. The 3,4-dibrominated furan compound of claim 7, wherein the furan-2,5-dicarboxylic dimethyl ester has a bio-renewable precursor.
 9. The 3,4-dibrominated furan compound of claim 8, wherein the bio-renewable precursor is a sugar.
 10. The 3,4-dibrominated furan compound of claim 8, wherein the bio-renewable precursor is an aldaric acid.
 11. A device comprising: a first electrode; a second electrode; and semiconducting layer disposed between the first electrode and the second electrode and including a photoactive layer, wherein the photoactive layer includes a semiconducting compound of the formula:

wherein each of R is selected from a group consisting of a C₁-C₂₀ linear alkyl chain, a C₂-C₂₄ branched alkyl chain, and a hydrogen atom; and wherein each of E is an electron deficient substituent selected from a group consisting of a bromoalkylthienyl-pyridylthiazole, a benzodithiazole, a pyridyldithiazole, a diketopyrrolopyrrole, a dithienopyrrolodione, a thienothiophene ester, a fluorinated thienothiophene ester, a dithienotetrazine, a thienoquinoxaline, a benzoquinoxaline, and a pyridylquinoxaline.
 12. The device of claim 11, wherein the first electrode and the second electrode comprise at least one material selected from a group consisting of a metal, a metal oxide, a metal alloy, a conductive polymer, Ag nanowires, Cu nanowires, graphene, carbon nanotubes, a polymer-metal hybrid, carbon-sulfur nanotubes, nanofibers, and an organic polymer.
 13. The device of claim 11, wherein the semiconducting layer is a bilayer.
 14. The device of claim 11, wherein the semiconducting layer is a bulk heterojunction.
 15. The device of claim 11, wherein the semiconducting layer further comprises a semiconducting material selected from a group consisting of fullerenes, a polymer, and a polymer blend.
 16. The device of claim 11, wherein the semiconducting compound is synthesized in a reaction between a dithienofuran core monomer and the electron deficient monomer.
 17. The device of claim 11, wherein the device is an organic photovoltaic cell.
 18. The device of claim 11, wherein the device is a field effect transistor.
 19. The device of claim 11, wherein the device is a flexible organic photovoltaic cell.
 20. The device of claim 11, wherein the device is a light sensor. 